1. Field of the Invention
The present invention relates to a method and device for activating regeneration of a nitric oxide adsorber.
2. Description of the Related Art
As is known, diesel engine emissions comprise the following compounds, some of which are harmful to health and/or the environment:                carbon dioxide (CO2) and steam (H2O), both produced by complete combustion of hydrocarbons in diesel fuel;        unburnt hydrocarbons (HC) and carbon monoxide (CO), both produced by incomplete combustion of hydrocarbons in diesel fuel;        nitric oxides (NOX) produced by oxidation of nitrogen in the engine air intake; and        particulate, mainly produced by incomplete combustion of the injected fuel.        
Carbon monoxide and hydrocarbons are convertible to carbon dioxide and steam by the following oxidation process, which is active when the air-fuel mixture is lean, i.e. high in oxygen:CO+HC+O2→CO2+H2O
Nitric oxides, on the other hand, are convertible to carbon dioxide, nitrogen, and steam by the following reduction process, which is effective when the air-fuel mixture is rich:NOx+CO+HC−>N2+CO2+H2O
Otto engines can exploit the above phenomena simultaneously.
To effectively eliminate all three of the above pollutants (HC, CO, NOX), the air-fuel mixture in the combustion chamber of Otto engines equipped with a trivalent catalyst must be stoichiometric, i.e. the amount of air fed into the combustion chamber must be the exact amount required to burn the fuel in the combustion chamber.
Trivalent catalysts, however, are unsuitable for use in diesel engines, operation of which calls for an air quantity in excess of stoichiometric proportions (lean operation), thus preventing reduction of nitric oxides in normal operating conditions, for the reasons stated above.
Nitric oxides produced by diesel engines, therefore, cannot be eliminated using a trivalent catalyst, and the effectiveness of a catalyst in this type of engine is limited solely to oxidizing carbon monoxide and hydrocarbons into carbon dioxide and steam.
In diesel engines, nitric oxides may be eliminated using a so-called nitric oxide adsorber, whereby, during normal operation of the engine, nitrogen monoxide (NO) is converted to nitrogen dioxide (NO2) by an oxidizing element, e.g. platinum (Pt), is then trapped in an adsorbent compound, e.g. barium oxide (BaO), and is separated, at a specific operating stage, into nitrogen and carbon dioxide by a reducing element, e.g. rhodium (Rh). The operating stage, known as regeneration, is achieved by calibrating the diesel engine to produce a reducing environment (rich operation) in the exhaust gas for a few seconds.
During the adsorption process, the barium oxide ultimately becomes unable to store nitrogen monoxide (NO), on account of saturation of the acceptor sites; and, when the nitric oxide adsorber reaches a saturation level at which nitric oxides are no longer eliminated effectively, the acceptor sites must be “cleared” periodically by so-called regeneration, i.e. desorption and simultaneous reduction of nitric oxides.
As stated, nitric oxide adsorption and deadsorption are closely related to the composition of the air-fuel mixture during operation of the engine. That is, to adsorb nitric oxides, the air-fuel mixture must be lean (i.e. oxidizing), whereas, to desorb and reduce nitric oxides, the air-fuel mixture must be rich (i.e. reducing).
More specifically, the nitric oxide adsorption and reduction mechanism commences in lean air-fuel mixture conditions, with oxidation of nitrogen monoxide (NO) into nitrogen dioxide (NO2) by the platinum (Pt) acting as a catalyst, according to the equation:NO+½O2−>NO2 
Subsequently, the nitrogen dioxide (NO2) reacts with the adsorbent element—barium oxide (BaO) in the example shown—by which it is trapped (i.e. chemically sorbed) in the form of barium nitrate (Ba(NO3)2), according to the equation:BaO+NO2+½O2−>Ba(NO3)2 
At the regeneration stage, the air-fuel mixture is enriched for a predetermined time period to increase carbon monoxide and unburnt hydrocarbon emissions and impart reducing properties to the exhaust gas.
The reducing atmosphere produces thermodynamic instability in the barium nitrate, which thus releases nitrogen monoxide (NO) and nitrogen dioxide (NO2), according to the equations:Ba(NO3)2−>BaO+2NO+½O2 Ba(NO3)2−>BaO+2NO2+½O2 
In rich air-fuel mixture conditions, and thanks to the presence of rhodium as a catalyst, nitrogen monoxide (NO) and nitrogen dioxide (NO2) are reduced by carbon monoxide (CO), hydrogen and hydrocarbons to nitrogen (N2) and carbon dioxide (CO2).
One possible reduction path is the equation:NO+CO−>½N2+CO2 
The success of the process depends directly on the air-fuel mixture, and, to effectively eliminate nitric oxides, the air/fuel proportion must be monitored continuously.
Efficient operation of a nitric oxide adsorber therefore substantially depends on the ability of barium to trap nitrogen dioxide, which in turn depends on various factors, such as exhaust gas temperature, exhaust gas flow, and the number of acceptor sites available in the barium. In the best possible operating conditions, a nitric oxide adsorber effectively eliminates as much as 90% of the nitric oxides produced by the engine.
One of the factors responsible for the reduction in adsorption capacity of barium is the presence of sulphur in the fuel. Unfortunately, at temperatures of over 300° C., sulphur oxidizes to sulphur dioxide (SO2), which in turn may be converted by humidity in the atmosphere to sulphur trioxide (SO3); which compounds react with barium oxide in the same way as nitrogen dioxide, i.e. tend to be trapped in the barium acceptor sites in the form of barium sulphate (BaSO4), so that some of the acceptor sites are permanently occupied by barium sulphate, thus preventing entrapment of part of the nitrogen dioxide, and so impairing adsorber efficiency. In fact, unlike regeneration of acceptor sites saturated with nitrogen dioxide, which occurs between 300 and 450° C., regeneration of sulphate-saturated acceptor sites requires temperatures of around 600° C.
To prevent sulphates damaging the nitric oxide adsorber, the fuel must therefore contain no sulphur or, to limit the extent of damage, must contain at most 10 ppm.
Though slow, sulphate accumulation in the adsorber is therefore inevitable, on account of small quantities being derived anyway from the lubricating oil as well as the fuel, and must be removed periodically, every 1000-4000 km, by a specific regeneration strategy combining reducing environment conditions and temperature levels of around 600° C.
The regeneration strategy of a nitric oxide adsorber is therefore clearly one of the major problems posed by automotive use of this type of catalyst.
For this purpose, various nitric oxide adsorber regeneration strategies have been proposed, which comprise a fixed-duration (about 60-second) accumulation step, during which the air-fuel mixture is lean—in particular, assumes a value (A/F) of 20 to 55—followed by a fixed-duration (about 5-second) regeneration step, during which the air-fuel mixture is enriched—in particular, assumes a value (A/F) of 12 to 14.
Controlling nitric oxide adsorber accumulation and regeneration cycles as described above, however, is unsatisfactory in terms of consumption and pollutant emissions, by regeneration possibly being performed when not strictly necessary, or, conversely, not being performed when actually required.
It is an object of the present invention to provide a method and device for activating regeneration of a nitric oxide adsorber, designed to eliminate the drawbacks of known methods.